1. Field of Invention
The present invention relates generally to a processing method for depositing porous films on a substrate. More specifically, the present invention relates to a processing method for depositing porous silica or doped silica films for fabricating semiconductor integrated circuits. The method is also advantageous for use in other applications where porous structures are required.
2. Description of Related Art
Traditionally, silicon dioxide, having a dielectric constant (k) about 4, is used as the insulator material for fabricating semiconductor integrated circuits. As device dimensions shrink, interconnect RC (resistance-capacitance) delay issues require the insulator to have a lower dielectric constant in order to deliver superior circuit performance. The semiconductor industry has identified these targets at various technology nodes and published them on the International Technology Roadmap for Semiconductors. Dielectric constant less than 4 is commonly referred to as low-k and that less than 2.2 is commonly referred to as ultralow-k. It is projected that beyond the 90 nm device generation, low-k dielectrics having a k-value below 2.6 is desirable for device fabrication.
The dielectric constant is a measure of the tendency of a material to allow an externally applied electric field to induce electric dipoles in the material. This so-called electric polarizability is governed by the electronic, ionic and distortion polarization in the material. A good review of the polarization phenomena and a more detailed description of the various classifications of low dielectric constant materials can be found in the article by K. Maex et al. [K. Maex, M. R. Baklanov, D. Shamiryan, F. Iacopi, S. H. Brongersma, and Z. S. Yanovitskaya, J. App. Phys., Vol. 93, No. 11, p. 8793–8841] or the chapter by S. Wolf [“Silicon Processing for The VLSI Era, Volume 4: Deep-Submicron Process Technology” by S. Wolf, Lattice Press, Sunset Beach, Calif., 2002, p. 639–670].
Fundamentally, to weaken the polarization in silicon dioxide, one can alter the structural lattice of silicon and oxygen, replace some or all of the silicon-oxygen bonds with less polarized bonds, and/or introduce free space to decrease material density in the film. Explored efforts include developing 1) silica-based doped oxides, 2) silsesquioxane-based inorganic-organic hybrid polymers, 3) organic polymers, and 4) amorphous carbon films.
Silica-based doped oxides are usually deposited by chemical vapor deposition (CVD) methods with or without plasma enhancement. Fluorine doping provides fluorosilicate glass (FSG) with a k-value about 3.6. Carbon or other alkyl substitution reduce the dielectric constant further; some reaching k-values as low as 2.6 to 2.8. An altogether amorphous carbon film or a fluorocarbon film has been reported to yield lower k-values. However, amorphous carbon technology is still very immature and for now is not ready for manufacturing considerations.
CVD silica-based doped oxides are appealing for use as semiconductor dielectrics due to their silicon-oxide like structure. The films require almost no modification in circuit designs. Semiconductor manufacturers can also leverage existing toolsets and infrastructures to continue their device fabrication. Some of these films have been adopted at the 180 nm, 150 nm, 130 nm, and even 90 nm nodes. However, the oxycarbide films are prone to carbon depletion in subsequent processing, resulting in a less than desired final dielectric constant. Furthermore, the incorporation of carbon in silica introduces many process complications, particularly in etching, chemical mechanical polishing, and cleaning. Consequently, implementation has been formidable and costly.
In contrast, silsesquioxane-based inorganic-organic hybrid polymers and organic polymers are inherently low-k dielectrics due to their more opened molecular lattice than silicon dioxide and less polarized bonds in the molecular components. These materials can provide a broad range of low k values. These films are usually applied by spin coating, although some can also be deposited by CVD methods. The spun film must go through curing to drive off excess solvent, complete the chemical reactions, and undergo densification. Compared to silicon dioxide, these films are generally mechanically softer and less thermally stable. They also tend to take up moisture so additional cap layers are often required to protect them. Because of the different properties, there are many restrictions in conventional processing and modifications are frequently needed to accommodate these films in process integration. Therefore, widespread adoption has not been noted.
Recently, the industry has come to conclude that there is no fully dense spin-on or CVD material that has a low enough dielectric constant and at the same time satisfy all the diverse requirements for robust integration for the 90 nm generation and beyond. Since the dielectric constant scales proportionally with the host matrix density, attention has been turned to exploring the viability of reducing the dielectric constant by introducing porosity in the insulator.
Sol-gel techniques are known to provide a flexible means for incorporating dopants and forming a porous template in silica networks. Sol-gel techniques, however, require meticulous gellation and drying. Their different modes of processing, process control, and integration schedules are incompatible with semiconductor device manufacturing. Many of these films also exhibit deterioration of mechanical properties with decreasing k-values.
A more adaptable approach to introduce void volume in the dielectrics has been the use of sacrificial porogens [see for example, U.S. Pat. No. 6,271,273 and U.S. Pat. No. 6,451,712]. A thermally unstable material, referred to as the porogen, is blended with an organosilicate polymer and applied to form a film as in conventional spin-on dielectrics. The film is cured, then subject to an annealing step to volatize the porogen while forming a skeletal porous framework of the cured film. Critical to this thermolytic technique are: First, the porogen must separate from the thermosetting matrix and must decompose as well as removed entirely during the annealing step without leaving behind any residue. Second, the porogen decomposition must take place below the host's glass transition temperature without bringing about any collapse of the porous structure. Third, change in film stress must be carefully managed during phase separation and thermal expulsion of porogens without causing any film cracking or delamination. Porous films formed with this method usually have a broad pore size distribution with the smallest pores in the 20 nm range.
The porogen concept has also been explored with CVD techniques [see for example, U.S. Pat. No. 6,054,206 and U.S. Pat. No. 6,171,945]. Thermally unstable labile organic groups are deposited in organosilicate glass. The film is then annealed to volatize the labile organic components, resulting in a porous structure. An alternative e-beam treatment [see for example, U.S. Pat. No. 6,737,365] or ultraviolet exposure [see for example, U.S. Pat. App. 20040096672] has also been reported to be effective in removing these species and additionally enhance cross-linking of the host material. In general, nanoporous films with pores commensurate in size of the departing organic groups are obtained with this approach. The nanoporous matrix is claimed to provide good mechanical and thermal stability in subsequent processing. However, like other porogen techniques, this CVD technique is based on volatilizing organic species, and there are concerns for residual outgassing if the organic species that are supposed to be removed are not removed entirely. Moreover, process integration complications associated with processing carbon-containing oxide films still remain, as discussed earlier.
To date, development of low-k films continues. The object of this invention is to create a CVD process method for generating a porous low-k dielectric film that is extendable to the ultralow-k range. It is desirable that the film is chemically, mechanically, and thermally stable, similar to that of silicon dioxide. It is further desirable that process integration requirements are not excessive and costly when compared to established techniques.